During the recording of the audio signals emitted by one or more sound sources in a space, multiple microphones may be utilized to capture the audio signals. In this regard, a first microphone may be placed near a respective sound source and a second microphone may be located a greater distance from the sound source so as to capture the ambience of the space along with the audio signals emitted by the sound source(s). In an instance in which the sound source is a person who is speaking or singing, the first microphone may be a lavalier microphone placed on the sleeve or lapel of the person. Following capture of the audio signals by the first and second microphones, the output signals of the first and second microphones are mixed. In the mixing of the output signals of the first and second microphones, the output signals of the first and second microphones may be processed so as to more closely match the long term spectrum of the audio signals captured by the first microphone with the audio signals captured by the second microphone. This matching of the long term spectrum of the audio signals captured by the first and second microphones is separately performed for each sound source since there may be differences in the types of microphone and the placement of the microphones relative to the respective sound source.
In order to approximately counteract the bass boost caused by placing a microphone with a directive pickup pattern, such as a cardioid or figure eight pattern, close to the sound source in the near field, a bass cut filter may be utilized to approximately match the spectrum of the same sound source as captured by the second microphone. Sometimes, however, it may be desirable to match the spectrum more accurately than that accomplished with the use of a bass cut filter. Thus, manually triggered filter calibration procedures have been developed.
In these filter calibration procedures, an operator manually triggers a filter calibration procedure, typically in an instance in which only the sound source recorded by the first microphone that is to be calibrated is active. A calibration filter is then computed based upon the mean spectral difference over a calibration period between the first and second microphones. Not only does this filter calibration procedure require manual triggering by the operator, but the operator generally must direct each sound source, such as the person wearing the first microphone, to produce or emit audio signals during a different time period in which the filter calibration procedure is performed for the first microphone associated with the respective sound source.
Thus, these filter calibration procedures are generally suitable for a post-production setting and not for the design of filters for live sound. Moreover, these filter calibration procedures may be adversely impacted in instances in which there is significant background noise such that the audio signals captured by the first and second microphones that are utilized for the calibration have a relatively low signal-to-noise ratio. Further, these filter calibration procedures may not be optimized for spatial audio mixing in an instance in which the audio signals captured by the first microphones associated with several different sound sources are mixed together with a common second microphone, such as a common microphone array for capturing the ambience, since the contribution of the audio signals captured by each of the first microphones cannot be readily separated for purposes of filter calibration.